Ionic multiresonant thermally activated delayed fluorescence emitters for light emitting electrochemical cells

We designed and synthesized two new ionic thermally activated delayed fluorescent (TADF) emitters that are charged analogues of a known multiresonant TADF (MR-TADF) compound, DiKTa. The emission of the charged derivatives is red-shifted compared to the parent compound. For instance, DiKTa-OBuIm emits in the green (λPL = 499 nm, 1 wt % in mCP) while DiKTa-DPA-OBuIm emits in the red (λPL = 577 nm, 1 wt % in mCP). In 1 wt % mCP films, both emitters showed good photoluminescence quantum yields of 71% and 61%, and delayed lifetimes of 316.6 μs and 241.7 μs, respectively, for DiKTa-OBuIm and DiKTa-DPA-OBuIm, leading to reverse intersystem crossing rates of 2.85 × 103 s−1 and 3.04 × 103 s−1. Light-emitting electrochemical cells were prepared using both DiKTa-OBuIm and DiKTa-DPA-OBuIm as active emitters showing green (λmax = 534 nm) and red (λmax = 656 nm) emission, respectively.


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Photophysical measurements. Optically dilute solutions of concentrations on the order of 10 −5 to 10 −6 M were prepared in spectroscopic or HPLC grade solvents for absorption and emission analyses. Absorption spectra were recorded at room temperature on a Shimadzu UV-2600 double beam spectrophotometer with a 1 cm quartz cuvette. Molar absorptivity determination was verified by linear regression analysis of values obtained from at least five independent solutions at varying concentrations ranging from 10 −5 M to 10 −6 M. Steady-state emission and excitation spectra and time-resolved emission decays were recorded at 298 K using an Edinburgh Instruments FS5 spectrofluorometer. Samples were excited at 378 nm for steady-state measurements and at 378 nm for time-resolved measurements.
For photoluminescence quantum yield measurements, degassed solutions were prepared via three freeze-pump-thaw cycles and spectra were taken using home-made Schlenk quartz cuvette. Photoluminescence quantum yields for solutions were determined using the optically dilute method [4] in which four sample solutions with absorbances of ca. 0.19, 0.15, 0.09, and 0.05 for DiKTa-OBuIm and ca. 0.21, 0.17, 0.10, and 0.05 for DiKTa-DPA-OBuIm were used. The Beer-Lambert law was found to remain linear at the concentrations of the solutions. For each sample, linearity between absorption and emission intensity was verified through linear regression analysis with the Pearson regression factor (R 2 ) for the linear fit of the data set surpassing 0.9. Individual relative quantum yield values were calculated for each solution and the values reported represent the slope obtained from the linear fit of these results. The quantum yield of the sample, ΦPL, can be determined by the equation Φ = (Φ * * * 2 2 ) 2 , where A stands for the absorbance at the excitation wavelength (λexc: 379 nm), I is the integrated area under the corrected emission curve and n is the refractive index of the solvent with the subscripts "s" and "r" representing sample and reference respectively. Φr is the absolute quantum yield of the external reference quinine sulphate (Φr = 54.6% in 1 N H2SO4) [5], The experimental uncertainty in the emission quantum yields is conservatively estimated to be 10%, though we have found that statistically we can reproduce PL values to 3% relative error.

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Spin-coated thin films were used to measure thin film photophysical properties in the solid state. An integrating sphere (Hamamatsu, C9920-02) was employed for photoluminescence quantum yield measurements for thin film samples. Time-resolved PL measurements of the thin films (film of emitters doped in mCP coated on sapphire substrate and annealed at 140 °C for 3 min) were carried out using the time-correlated single photon counting system (TCSPC) and multichannel scaling (MCS). The film samples were excited at 379 nm by a pulsed laser diode (Picoquant, model PLS 370) and were kept in a vacuum of < 8 × 10 −4 mbar. Fitting of the time-resolved luminescence measurements and relative rates: Time-resolved PL measurements were fitted to a sum of exponentials decay model, with chi-squared (χ 2 ) values between 1 and 2, using the EI software. Each component of the decay is assigned a weight, (wi), which is the contribution of the emission from each component to the total emission.
The average lifetime was then calculated using the following:  Two exponential decay model: with weights defined as 1 = where A1 and A2 are the preexponentialfactors of each component.
 Three exponential decay model: with weights defined as 1 = 1 1 where A1, A2 and A3 are the preexponential-factors of each component.
Vertical excited states were performed based on the ground state optimised structure using spin-component scaling coupled-cluster singles-and-doubles model (SCS-CC2) with the cc-pVDZ basis set [12], computing the two first singlet (S1 and S2) [13] and two first triplet excited states (T1 and T2) [14].
Molecular orbitals were visualised using GaussView 6.0 [15]. Difference density plots were used to visualise change in electronic density between the ground and excited state and were obtained using the S5 VESTA package [16]. Charge transfer descriptors, distance of charge transfer (DCT) was calculated from the difference density plots using Multiwfn software package [17]. The characterisation was conducted under nitrogen atmosphere.